There are two methods that have been used on this forum.
1. Remove the wood interior trim piece. Drilling from the bottom up, drill thru the piece of wood. Run bolts from the inside down (counter sink the holes & use washers. Install the bracket using nylon insert nuts. Reinstall the wood trim piece in interior.
2. If the wood is rotted that the bracket is mounted to, add a length of angle iron to the bottom side of the top. Drill & tap holes in the angle iron to mount the bracket. Hope either of the two methods work for you.
Bob